This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-310298, filed Oct. 29, 1999; and No. 2000-166059, filed Jun. 2, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a sputtering apparatus and film forming method and, more particularly, to a sputtering apparatus effective for forming an interconnection film made of Al or Cu, a barrier metal film made of TaN or TiN, and a liner film made of Ti or Nb, and a film forming method using this apparatus.
In the semiconductor process, a sputtering apparatus is widely used as a film forming apparatus. One of the reasons for this is that the sputtering apparatus requires a low running cost while providing a high productivity.
As semiconductor devices are recently micro-patterned and highly integrated, the aspect ratio (the ratio of depth to opening diameter) of a contact hole tends to increase. For example, when an interlevel insulating film is to be deposited on an Si substrate with a surface having a diffusion layer and a contact hole to be connected to the diffusion layer is to be formed in this interlevel insulating film, this contact hole has a high aspect ratio. Therefore, for example, when a Ti silicide layer is to be formed on the surface of the diffusion layer, it is not easy to form a thick Ti film on the bottom surface of the contact hole by sputtering. When the inner surface (side and bottom surfaces) of the contact hole is to be covered with a barrier metal film such as a TiN film, or a glue layer for W-CVD, the barrier metal or the like cannot be easily formed on the entire inner surface of the contact hole to a uniform thickness by sputtering.
The Al reflow technique is known as a technique of filling a contact hole with an Al film. Of the Al reflow technique, a 2-step reflow scheme of sequentially forming a liner film, a first Al film, and a second Al film by sputtering has become the mainstream. According to the 2-step reflow scheme, the first Al film is formed by cooling, while the second Al film is formed while heating. When forming the second Al film, Al flows in the contact hole through the first Al film as the diffusion path. Therefore, the first Al film must be formed on the entire inner surface of the contact hole.
A film forming method using the conventional sputtering apparatus has poor step coverage. Accordingly, it is not easy to increase the thickness of the film on the bottom surface of the contact hole, to uniform the thickness of the film on the entire inner surface of the contact hole, or to form the film on the entire inner surface of the contact hole.
Conventionally, an Al interconnection is often used as an LSI interconnection. In recent years, an interconnection structure as a combination of an insulating film with a low dielectric constant and a Cu interconnection has been studied. This is sought for in order to decrease the resistance and increase the reliability of the interconnection, i.e., in order to improve RC delay and improve the EM resistance. Since compounds of Cu that have a high vapor pressure are few, Cu is difficult to process by RIE (Reactive Ion Etching). Accordingly, it is difficult to form an RIE interconnection from Cu. Hence, when forming a Cu interconnection, the damascene process that does not use RIE is the mainstream.
In the damascene process, a metal film is formed by deposition on the entire surface to fill an interconnection groove formed in an interlevel insulating film in advance. After that, an excessive portion of the metal film outside the interconnection groove is removed by CMP (Chemical Mechanical Polishing) to form an interconnection (damascene interconnection) formed of the metal film. In particular, a process of forming a groove and contact hole in an interlevel insulating film in advance and filling the groove and contact hole with a metal film at once, thereby forming an interconnection and plug simultaneously is called a dual damascene process (DD process).
When a Cu interconnection is to be formed by the damascene process, a Cu film is naturally used as the metal film. As Cu tends to diffuse in the interlevel insulating film, Cu in the Cu film diffuses to the Si substrate. Cu diffused to reach the Si substrate forms a deep level in Si. This deep level traps carriers to degrade the element characteristics.
For this reason, when a Cu interconnection is to be formed by the damascene process, a barrier metal film diffusion preventive film) for preventing diffusion of Cu is formed by sputtering on the inner surface of the interconnection groove before the Cu film is deposited. With the DD process, a barrier metal film must also be formed on the inner surface of the contact hole. As the barrier metal film, a TIN film, a TaN film, and the like are widely studied. The barrier metal film is desirably formed uniformly on the entire inner surface of the interconnection groove, or the entire inner surfaces of the interconnection groove and contact hole because of its purpose.
In the DD process for Cu, the interconnection groove and the like must be filled with a Cu film. As a Cu film forming method, electroplating is widely studied. Cu electroplating requires a seed layer for supplying electrons to electroplating solution. The barrier metal film, such as the TaN film, described above however does not function well as a seed layer. For this reason, after the barrier metal film is formed, a seed layer is often formed by sputtering Cu itself as the material of the interconnection. Such a seed layer (Cu seed layer) is desirably formed uniformly on the entire inner surface of the interconnection groove, or the entire inner surfaces of interconnection groove and contact hole because of its purpose.
To meet these requirements, a sputtering apparatus, e.g., a long throw sputtering apparatus or ionization sputtering apparatus, which has an improved sputtering particle directivity is used to form a Ti film, TiN film, TaN film, and Cu seed layer.
FIG. 22 is a schematic view showing a conventional long throw sputtering apparatus. In the long throw sputtering apparatus, the distance between a substrate 81 and target 82 is increased, and an Ar pressure is decreased to suppress gas scattering. This suppresses oblique incident components of the sputtering particles into the contact hole, thereby improving the directivity of the sputtering particles.
FIG. 22 shows a process chamber 83, a magnet 84, a backing plate 85, cooling water paths 86, an insulating member 87, an earth shield 88, a first shield plate 89 to prevent deposition on the other portions (to be referred to as a shield plate), a second shield plate 90, an elevating system 91 of the second deposition preventive plate 90, an electrostatic chucking plate 92, a susceptor 93 (worktable), a coolant 94, a DC voltage source 95 for applying a voltage to the target 82, and a pair of ring-like magnets 84a and 84b. 
A film forming method using the conventional long throw sputtering apparatus has the following problems more specifically, the shape of the formed film becomes asymmetric at the end of the substrate. A metal film is accordingly difficult to form uniformly, and the coverage is poor.
In the ionization sputtering apparatus, an RF (Radio-Frequency) power is introduced to an induction coil attached between the target and substrate, thereby generating a high-density plasma of Ar gas supplied into the process chamber. The sputtering particles are ionized in the high-density plasma, and a negative voltage is applied to the substrate, thereby improving the directivity of the sputtering particles.
The film forming method using the conventional ionization sputtering apparatus has the following problems. During film formation, Ar as the sputter gas is also ionized in addition to the sputtering particles. Hence, ionized sputter particles and Ar are attracted to the substrate. The Ar ions attracted to the substrate collide against the substrate. This collision transforms the kinetic energy of Ar ions into heat to increase the substrate temperature.
Al and Cu are metals that tend to agglomerate easily. The higher the substrate temperature, the more likely agglomeration occurs. If an Al or Cu film is formed by the ionization sputtering apparatus, the Al or Cu film agglomerates and is separated easily. Therefore, if the first Al film in the 2-step reflow scheme of Al is formed by using the ionization sputtering apparatus, it causes agglomeration and the diffusion path for Al fluidization disappears, so the contact hole cannot be filled with the second Al film. If the Cu seed layer in electroplating is formed by using the ionization sputtering apparatus, it causes agglomeration and electric conduction for Cu plating cannot be obtained, so the contact hole cannot be filled with a Cu film by electroplating.
Another example of the sputtering apparatus having Cu directivity utilizes self-sustained plasma of Cu. With Cu, sputtered Cu is ionized under specific voltage, current, and magnetic field conditions even when supply of Ar serving as the sputter gas is stopped. Ionized Cu itself collides against the target to force out Cu particles, thereby sustaining plasma.
This self-sustained plasma is not limited to Cu but occurs with other metals as well. Since Cu is a metal that tends to cause self-sustained plasma particularly easily, sputtering by means of self-sustained plasma of Cu has conventionally long been studied (Asamaki: Basic Thin Film Fabrication (third edition) (THE NIKKAN KOGYO SHINBUN. LTD), pp. 195-242, Sano et al: Extended Abstract (The 40th Spring Meeting, 1993) of The Japan Society of Applied Physics and Related Societies, No. 2, p. 393, and Horiike et al: Jpn. Pat. Appln. No. 5-257512 (Japanese Patent No. 2,914,644, Jpn. Pat. Appln. KOKAI Publication No. 7-94413 (Apr. 7, 1995))).
Sufficient directivity cannot be obtained with self-sustained sputtering of Cu alone, and if the contact hole has an aspect ratio of 3 or more, the step coverage becomes poor. Therefore, a sputtering apparatus as a combination of self-sustained sputtering and a scheme for increasing the distance between the sputter target and substrate, as in long throw sputtering described above, has been studied (Horiike et al: Jpn. Pat. Appln. No. 8-91278 (Jpn. Pat. Appln. KOKAI Publication No. 9-256149 (Sep. 9, 1997)), and Kotani et al: Extended Abstract (The 57th Autumn Meeting, 1996) of The Japan Society of Applied Physics and Related Societies, No. 2, p. 642).
With a sputtering apparatus of this type, since self-sustained sputtering is utilized, Ar gas need not be supplied, and scattering of sputter particles caused by Ar gas does not occur at all. However, even if self-sustained sputtering and long throw sputtering are combined, the asymmetry of the shape of the formed film, which is the fundamental problem of long throw sputtering, is not solved, and becomes rather conspicuous as gas sputtering does not occur. It is therefore practically difficult to form a Cu film on an Si substrate with a diameter of 200 mm or more by using a sputtering apparatus utilizing conventional self-sustained sputtering and long throw sputtering.
Furthermore, in the sputtering apparatus utilizing self-sustained plasma of Cu, it has been studied to apply a DC negative voltage to the substrate so that Cu ions are attracted to the substrate with a good directivity (A. Sano et al: Advanced Metallization and Interconnect Systems for ULSI Applications in 1995, pp. 709-715, and Tsukada et al: Jpn. Pat. Appln. No. 53-57812 (Jpn. Pat. Appln. KOKAI Publication No. 54-149338 (Nov. 22, 1979)), Sano et al: Extended Abstract (The 42nd Spring Meeting, 1995) of The Japan Society of Applied Physics and Related Societies, No. 2, p. 813, Sano et al: Extended Abstract (The 56th Autumn Meeting, 1995) of The Japan Society of Applied Physics and Related Societies, No. 2, p. 607, and Sano et al: Extended Abstract (The 43rd Spring Meeting, 1996) of The Japan Society of Applied Physics and Related Societies, No. 2, p. 747).
In self-sustained sputtering, when the directivity of Cu ions is to be increased by applying a DC negative voltage to the substrate, ions that are attracted to the substrate by the DC negative voltage are only Cu ions. Hence, unlike in the case using the ionization sputtering apparatus, the substrate temperature is not increased by Ar ions, and an increase in substrate temperature can be suppressed to the necessary minimum level.
The sputtering apparatus of this type has the following problems. In self-sustained sputtering of Cu, many Cu ions are restrained near the target by the target voltage and the magnetic field, and the number of free ions to be attracted by the negative voltage applied to the substrate is small. The Cu ions accordingly do not have large effect on film formation, and excellent step coverage cannot be obtained. If Cu ions near the target are to be further attracted to the substrate, the target current density decreases, and self-plasma may not be sustained.
In self-sustained sputtering of Cu, it has been proposed to ionize Cu neutral particles in order to increase the number of Cu ions, so that more Cu ions are attracted to the substrate upon application of the DC negative voltage to the substrate. More specifically, an attempt has been proposed to introduce an RF power to the induction coil between the target and substrate so as to form a high-density plasma, so that Cu neutral particles are ionized in the high-density plasma (Horiike et al: Jpn. Pat. Appln. No. 8-91728, and Itsuki et al: Extended Abstract (The 43rd Spring Meeting, 1996) of The Japan Society of Applied Physics and Related Societies, No. 2, p. 748).
Different from ionization sputtering of supplying an inert gas such as Ar, self-sustained sputtering is performed in a high vacuum. For this reason, even when an AC current with a very high frequency is introduced, it is difficult to ionize Cu neutral particles traveling in vacuum. Therefore, even when self-sustained sputtering and ionization of Cu neutral particles are combined, a sufficiently high-density plasma cannot be formed, i.e., the number of Cu ions contributing to formation of the Cu film cannot be increased, and a practical sputtering apparatus is difficult to realize.
Self-sustained sputtering has a drawback in that its film formation rate is excessively high. To improve the film quality, it is generally effective to increase the film formation rate. If, however, the film formation rate is excessively high, a thin film is difficult to form with good controllability, as described in Jpn. Pat. Appln. No. 5-257512.
In most of the recent sputtering apparatuses, a magnet is arranged above the target to increase the plasma density. It is difficult to increase the plasma density on the entire surface of the target, and in most cases, a high-density plasma region is shifted to sputter the entire surface of the target. In this case, the magnet above the target is often rotated. An increase in rotational speed of the magnet is limited. If the film formation rate is excessively high, sputtering is ended while the rotational number is still small. Consequently, variations in film thickness distribution within the substrate surface may increase, or the controlled film thickness may vary, posing a problem in the production.
In self-sustained sputtering, the film formation rate tends to increase because discharge cannot be sustained unless a high target voltage and large current, and a high magnetic flux density of the target surface are maintained. According to Jpn. Pat. Appln. No. 53-57812, to continue self-sustained sputtering of Cu, the target current density must be 130 mA/cm2 or more. According to Jpn. Pat. Appln. No. 5-257512, to form a Cu film with a thickness of 0.5 xcexcm to 1 xcexcm by self-sustained sputtering of Cu at a controllable film formation rate, the target current density must be 100 mA/cm2 or less. Although these two numerical values are contradictory, it may depend on the magnetic flux density on the target surface, the target shape, and the like.
Jpn. Pat. Appln. No. 5-257512 does not describe the lower limit of the target current density necessary for sustaining self-plasma. If the target current density is merely decreased in order to decrease the film formation rate, it becomes close to the lower limit of self-sustained discharge, as described in Jpn. Pat. Appln. No. 53-57812, and discharge may be interrupted and thus become unstable.
In the recent Cu damascene process, as described above, the main stream is the method of covering the inner surfaces of the interconnection groove and contact hole with a Cu seed layer by sputtering and thereafter filling the interconnection groove and contact hole with a Cu film by electroplating. In this case, a plating solution must enter the interconnection groove and contact hole. Therefore, the openings of the interconnection groove and contact hole should not be narrowed by the Cu seed layer.
Since the integration degree of the recent LSIs increases, many LSIs have interconnection grooves and contact holes with widths and diameters of 0.3 xcexcm or less. If a thick Cu seed layer is formed on the inner surfaces of these fine interconnection groove and contact hole, the openings of the interconnection groove and contact hole become narrow. Thus, the Cu seed layer must be formed thin, and more specifically, preferably has a thickness of 0.3 xcexcm or less.
In the target current density region with a target current density of 100 mA/cm2 or less, disclosed in Jpn. Pat. Appln. No. 5-257512, which is equal to or more than the lower limit that can sustain self-discharge, since the film formation rate is high and self-sustained discharge is not stable, a thin seed layer described above is difficult to form.
In self-sustained sputtering of Cu, a method has been proposed of applying a positive voltage to around the susceptor, thereby controlling the ionization energy and the directivity of ions (U.S. Pat. No. 5,897,752 corresponding to Jpn. Pat. Appln. KOKAI Publication No. 11-100668). According to the content disclosed in this reference, a component for applying the positive voltage becomes an anode to be electrically coupled with the cathode, so the directivity of the ions is difficult to control, as will be described later in the embodiments of the present invention. Accordingly, this method cannot solve the problem of poor step coverage described above.
Jpn. Pat. Appln. No. 7-40182 (Japanese Patent No. 2,912,181, Jpn. Pat. Appln. KOKAI Publication No. 8-239761 (Sep. 17, 1996)) shows a structure in which the potential of the shield or the inner wall of a vacuum vessel is set at a positive value to prevent inert gas ions from attaching to the inner wall of the vacuum vessel. With the disclosed contents alone, however, a component for applying the positive voltage becomes an anode to be electrically coupled with the cathode, so the directivity of the inert gas ions is difficult to control, i.e., the inert gas ions are difficult to prevent from attaching to the inner wall of the vacuum vessel.
As described above, sputtering apparatuses, e.g., a long throw sputtering apparatus and an ionization sputtering apparatus, which sustaining self-plasma have been conventionally known. All the sputtering apparatuses have a problem in coverage (asymmetry of the shape of the formed film at the substrate end and the step coverage at the contact hole). Furthermore, in the sputtering apparatus sustaining self-plasma, the film formation rate is excessively high.
The present invention has been made in consideration of the above situations, and has as its object to provide a sputtering apparatus and a film forming method that enable film formation with good coverage.
According to a first aspect of the present invention, there is provided a sputtering apparatus comprising:
a vacuum process chamber configured to accommodate a substrate to be processed;
a support member configured to support the substrate in the process chamber;
a gas supply system configured to supply a process gas into the process chamber;
an exciting system configured to excite the process gas to generate a plasma by causing discharge in the process chamber;
a sputter target disposed in the process chamber so as to oppose the substrate and to be collided by ions in the plasma to emit a film forming material;
a first negative biasing section configured to apply a first negative potential to the target;
a second negative biasing section configured to apply a second negative potential to the substrate;
an ion reflecting plate disposed in the process chamber so as to surround a space between the substrate and the target;
a positive biasing section configured to apply a positive potential to the ion reflecting plate;
a conductive component disposed in the process chamber to form a path along which electrons are released from the plasma; and
a magnetic field forming system configured to form in the process chamber a closed magnetic field for trapping electrons in the plasma on a surface of the target and a divergent magnetic field for directing the electrons in the plasma to the conductive component.
According to a second aspect of the present invention, there is provided a sputtering apparatus comprising:
a vacuum process chamber configured to accommodate a substrate to be processed;
a support member configured to support the substrate in the process chamber;
a gas supply system configured to supply a process gas into the process chamber;
an exciting system configured to excite the process gas to generate a plasma by causing discharge in the process chamber;
a sputter target disposed in the process chamber so as to oppose the substrate and to be collided by ions in the plasma to emit a film forming material;
a first negative biasing section configured to apply a first negative potential to the target;
a second negative biasing section configured to apply a second negative potential to the substrate;
an ion reflecting plate disposed in the process chamber so as to surround a space between the substrate and the target;
a positive biasing section configured to apply a positive potential to the ion reflecting plate;
a conductive component disposed in the process chamber to form a path along which electrons are released from the plasma; and
a control mechanism configured to set an electric resistance between the target and the ion reflecting plate to be higher than that between the target and the conductive component when the plasma is formed and the ion reflecting plate and the conductive component are grounded.
According to a third aspect of the present invention, there is provided a film forming method using the apparatus according to claim 1, comprising the steps of:
accommodating the substrate to be processed in the process chamber and supporting the substrate on the support member;
supplying the process gas into the process chamber;
applying the positive potential to the ion reflecting plate so that the ions of a film forming material are reflected and guided to the substrate;
exciting the process gas to generate the plasma by causing discharge in the process chamber;
applying the first negative potential to the target so that ions in the plasma collide against the target to emit the film forming material;
exciting the film forming material with the electrons in the plasma trapped by the closed magnetic field to ionize at least part of the film forming material;
flowing the electrons in the plasma with the divergent magnetic field to the conductive component and releasing the electrons from the process chamber through the conductive component, so that the conductive component serves as an anode with respect to the target; and
forming a film on the substrate with the film forming material while attracting the ions of the film forming material to the substrate by applying the second negative potential to the substrate.
According to the present invention, even ions that cannot come directly incident on a substrate to be processed can be reflected by an ion reflecting plate to come incident on the substrate. In contrast to this, according to the prior art, ions that cannot come directly incident on the substrate disappear in the grounding member and cannot contribute to film formation. Therefore, according to the present invention, the number of ions contributing to film formation can be increased, and film formation can be performed with better coverage than in the prior art.
The sputtering apparatus described in Jpn. Pat. Appln. No. 7-40182 is similar to the sputtering apparatus of the present invention as it has a component charged with a positive potential in it. According to Jpn. Pat. Appln. No. 7-40182, a portion charged with a positive potential is formed in the sputtering apparatus to prevent dust or mixing of impurities there.
According to the content described in Jpn. Pat. Appln. No. 7-40182, a component which applies a positive voltage is likely to become an anode to be electrically coupled with the cathode, and accordingly the directivity of the ions is difficult to control, as will be described later in the embodiments of the present invention. Jpn. Pat. Appln. No. 7-40182 does not disclose a mechanism that can apply a negative voltage to the substrate, and ions produced from the target material cannot be attracted to the substrate. Due to these differences in arrangement, the sputtering apparatus described in Jpn. Pat. Appln. No. 7-40182 cannot improve the step coverage.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.